This invention relates to a grinding method using the fine-particle electrophoresis phenomenon, and to a glass lens worked by the grinding method.
A grinding method using the electrophoresis phenomenon is known from, for example, document "Research Concerning Grinding Method Using Electrophoresis Phenomenon of Ultra-fine Particles" published in a 1996 spring convention of The Japan Society for Precision Engineering.
The document describes a grinding device for grinding an object or workpiece 25 so that its surface becomes flat, which comprises, as is shown in FIG. 17, a cup-shaped grinding stone 20 rotatable about its axis of rotation, mounted on an air spindle 30 which is movable along the axis of rotation of the grinding stone, and having a cylindrical portion and a disk-shaped portion; an electrode 21 provided with a predetermined distance from a ring-shaped working end surface of the cylindrical portion of the grinding stone 20; a DC power 22 connected to the electrode 21 and the air spindle 30 such that the electrode and the grinding stone serve as a cathode and an anode, respectively; means 23 for supplying, between the electrode and the stone, a grinding solution with silica fine particles (colloidal silica) 24 dispersed therein; and a sample table 26 opposed to the ring-shaped working end surface and disposed to mount the object 25 thereon.
While in the above grinding device, the grinding solution is supplied between the electrode 21 and the grinding stone 20, negative and positive voltages are applied to the electrode 21 and the grinding stone 20 from the DC power 22, respectively, thereby electrically attaching, to the surface of the grinding stone, silica fine particles which have been charged with negative electricity. Thus, a silica fine-particle layer is formed on the grinding stone surface, as a result of the electrophoresis phenomenon. In this state, the grinding stone 20 is gradually moved along the axis of rotation, and the silica fine-particle layer is brought into contact with the to-be-worked surface of the object. At the same time, the grinding stone 20 is rotated about the rotation axis to thereby make silica fine particles serve as a grinding blade for grinding the object. As a result, the object surface is polished into a mirror surface with little damage.
The above-described grinding method is effective in a case where the to-be-worked surface of the object has beforehand a certain shape (which is not a final surface shape or a surface of a mirror state), and is polished into a mirror surface by slightly removing material therefrom using silica fine particles. For example, the method is effective where only a very thin or small portion of a material has to be ground as in the case of a semiconductor wafer, and it is necessary to minimize the degree of deformation inside the worked material.
However, since in the above-described prior case, the electrical force for holding silica fine particles on the grinding stone is much smaller than the force for grinding the material, the fine particles will fall from the grinding stone if deep cuts are formed in the grinding stone to create a great working force.
In light of this, it is necessary to set the depth of cuts in the grinding stone at an extremely low value of several microns or less, in order to prevent falling of silica fine particles from the stone and to effectively use them as grinding particles.
Therefore, grinders having cuts with a depth of several microns or less are not effective in deeply grinding a workpiece, for example, to generate an optical element such as a lens from a glass blank (an optical glass workpiece). Since the cutting amount of the grinders is extremely small, efficient grinding cannot be performed, and hence an extremely long cutting time is required. This being so, it is necessary in the prior technique to beforehand prepare a material which has its to-be-worked surface ground into as close a shape as possible to the final shape, using another polishing or grinding device. Thus, lots of time is necessary for preparation of such a half product or for generation of a mirror surface from the workpiece or material.